1. Field of the Invention
This invention relates to a new and improved magnetic damping system for producing magnetic braking or retarding torque on a conductive disk of an induction device and more particularly to such a damping system including a low flux density permanent magnet having a pair of flux concentrating pole pieces for efficiently directing braking magnetic fields into the meter disk.
2. Description of the Prior Art
Magnetic braking or eddy current brake arrangements are commonly included in induction type meter and relay devices and in particular in induction electromechanical watthour meters. An electromagnetic unit having voltage and current sections are typically included in the induction watthour meters for connection between an electrical source and a load for measuring the consumption of AC electric energy. AC magnetic fields from the electromagnetic unit produce a driving torque on a rotatable armature formed by an electroconductive disk. Interaction between the changing magnetic fields and changing eddy currents induced in the disk by the fields develops a metering responsive driving torque on the disk. Associated with the measuring rotation of a watthour meter is a magnetic brake or damping system for directing a unidirectional or permanent magnet braking field into the disk which induces eddy currents. The braking magnetic field reacts with the eddy currents to produce a retarding torque on the disk. The retarding torque is proportional to the disk speed and balances the disk driving torque. The disk speed is maintained accurately proportional to the electric power applied through the meter so that each disk rotation is representative of a predetermined quantum of electric energy consumption.
The braking magnetic field of most modern watthour meters is provided by permanent magnets which direct magnetic fluxes through an air gap space receiving the meter disk. The strength and stability of the permanent magnets, the position of the braking magnetic field in relation to the disk center, the area, shape and flux density of the braking field entering the disk, and the length of the air gap spacing are all important factors in controlling and producing the retarding torque. It is essential that the retarding torque be kept proportionally constant for efficient and accurate meter operation over very long meter lifetimes, and while the meter operation is subject to widely varying temperature and atmospheric changes. Substantial mechanical shock and vibration can occur in shipping and handling and the meter is also subject to strong demagnetizing effects caused by electrical surges through the meter due to lightning and other causes.
The design of a magnetic damping system must incorporate the above and other factors and considerations including an assembly that is easily and simply manufactured at minimum cost in accordance with high volume production techniques. The system must have a size and configuration capable of being incorporated and mounted within the small space of a watthour meter available for such systems. The damping permanent magnets must be capable of being magnetized and demagnetized to establish predetermined levels of magnetization and induction while assembled in the system. Provision for calibration of the retarding torque must be further included and is referred to as full load adjustment in the watthour metering art. Temperature compensation is further required to compensate for reversible temperature-related changes in the remanence or magnetization of the damping permanent magnets and in the temperature dependent operating characteristics of the meter electromagnet unit producing the disk driving torque, referred to as Class 1 temperature errors in the watthour metering art.
Since permanent magnets form the principal element of a damping system, the magnetic characteristics of the permanent magnets principally control the system design configuration required to produce the desired eddy current brake and disk retarding torque. It is generally desirable to utilize a permanent magnet having highly anisotropic, high energy, high magnetic flux density and high coercive magnetic characteristics. Also desirable are relatively high temperature and life-time stability characteristics, availability in useful and compact shapes and sizes, and further having costs within commercially acceptable amounts consistent with total manufacturing cost.
In general, permanent magnetic materials are characterized as "hard" magnetic materials since they are difficult to magnetize; but once they are magnetized, they remain in the state of magnetization due to having a high coercivity or coercive force (Hc) characteristic. The residual flux density or induction (Br) and remanence characteristic of permanent magnet materials produce the operating flux density (Bd) in the magnet and externally at the magnet ends. An air gap flux density (Bg) and total magnetic field strength in the air gap of an eddy current brake gap is then dependent upon the magnet characteristics.
One extensively and commercially used C-shaped damping permanent magnet for one type of watthour meter is described and claimed in U.S. Pat. Nos. 3,309,152 and 3,076,934, both assigned to the assignee of this invention. Another commercially used magnetic damping assembly for another type of watthour meter has a pair of bar damping magnets spaced apart to form an air gap wherein a meter disk rotates as described in U.S. Pat. No. 3,688,192, also assigned to the assignee of this invention. The meter magnetic damping systems of the aforementioned patents utilize permanent magnets of a commercially available Alnico 5 and Alnico 8 permanent magnet materials having suitable high strength or high density magnetic flux with relatively high coercive force magnetic characteristics. In U.S. Pat. application Ser. No. 903,324 filed May 5, 1978 and assigned to the assignee of this invention, a magnetic damping system for watthour meters is described and claimed using a pair of compact and substantially flat permanent magnets which are opposingly spaced to form an eddy current brake air gap. The permanent magnets are made of a more recently developed cobalt-rare earth permanent magnet material having high energy, high coercivity and residual induction, and highly anisotropic characteristics. In U.S. Pat. No. 4,030,031 a meter magnetic damping system is also disclosed using a pair of spaced, compact and substantially flat permanent magnets also made of a cobalt-rare earth permanent magnet material.
Many permanent magnet materials and compositions including those described for the meter magnet magnetic damping systems in the aforementioned application and patents, include critical chemical elements in their chemical compositions which are becoming or are expected to become increasingly scarce or substantially more costly so that their use becomes increasingly unattractive or unavailable. At least two such potentially critical chemical elements are cobalt and nickel which are used in the Alnico, rare earth and other permanent magnet materials included in moderate to high energy permanent magnets having high residual flux densities. Alternatively, most of the other available permanent magnets not including critical chemical elements in their chemical compositions also do not have the high strength and flux density, high coercivity, stability or suitable physical characteristics including volume or shape required for magnetic damping. When it is desired to utilize a permanent magnet, not including one of the critical elements, which provide one or more of the enhanced magnetic characteristics, the damping system design must achieve the most efficient use of and compensation for one or more of the deficient magnetic characteristics of the alternative permanent magnets which may be more available or have substantially lower or more desirable costs.
Many permanent magnets are limited to certain shapes and geometrical configurations by the magnet energy-product characteristics and the physical and chemical material characteristics, including those contemplated for use with the present invention. Many magnets are made with powder metallurgical techniques incorporating use of high pressures and temperatures in sintering and bonding processes so that they are normally available only in regular straight-line bar, block or slab shapes and sizes. Conversely, the C-shaped permanent magnet of the aforementioned U.S. Pat. Nos. 3,309,152 and 3,076,934 is made by a casting or molding technique used in Alnico 5 magnets. In the other of the aforementioned patents and in U.S. Pat. No. 1,705,682, vertically magnetized slab or short block and horizontally elongated and magnetized bar permanent magnets are disclosed. As so far as is known, the magnets of most damping systems of modern induction devices including watthour meters include materials having at least one critical element such as cobalt.
U.S. Pat. No. 817,305 discloses an older type magnetic damping system including a large U-shaped permanent magnet made of one of older low energy type magnet materials. The system includes a soft magnetic strap circumscribing the magnet to increase efficiency by capturing and utilizing excess stray or leakage magnetic fields, characteristic of early magnets having low anisotropy, and applying them to opposing pole tips forming a damping air gap. U.S. Pat. No. 2,309,414 discloses a pair of short block permanent magnets in a meter magnetic damping system both disposed vertically and on one side of a disk air gap. The permanent magnets are disposed in oppositely poled relationships and include a soft magnetic yoke extending from the outer magnetic pole faces to the other side of the air gap. British Pat. No. 906,404 discloses a pair of horizontally disposed bar damping permanent magnets that are long and have small cross sections. Each magnet has opposite curved or bent soft magnetic pole pieces. The horizontally magnetized and elongated magnets have magnetic fluxes that are bent and directed through more than a ninety degree arc by the pole pieces through a high flux leakage path and across a vertical air gap including a meter disk. The high flux leakage is offset by the resultant closely spaced braking magnetic fields that are intended to avoid error producing interference with the AC metering fields.
The aforementioned patents also include disclosures of various Class 1 temperature-compensating arrangements usually disposed along the side of the magnets and these are not highly efficient for high anisotropic magnets. The aforementioned patents also disclose various full-load adjusting features and arrangements which often include a screw-type adjuster for varying permeance in leakage or flux return paths of the damping system.